This invention relates to a high strength ceramic honeycomb structure suitable for use in catalyzer carriers for purifying exhaust gases from automobile engines, or heat exchangers or deodorizers used in factories or homes.
Ceramic honeycomb structures have been widely used for catalyzer carriers for purifying automobile engine exhaust gases or heat exchangers. As the ceramic honeycomb structures are usually secured to insides of casings through ceramic mats or wire meshes and are used at high temperatures, strengths in radial directions on side surfaces (referred to hereinafter "isostatic strength") are more important than those in axial directions. Most of the honeycomb structures in actual cases include quadrilateral, particularly square cells owing to the fact that the honeycomb structures including the quadrilateral cells are easy to be manufactured and serve to reduce pressure losses. These honeycomb structures exhibit fairly high isostatic strength in directions perpendicular to the thickness of partitions of honeycomb structures. However, their isostatic strength in directions oblique to the partitions is very low. Therefore, there has been a risk of the honeycomb structures being broken off in diagonal directions in casings due to thermal stresses in use or forces mechanically acting upon the honeycomb structures being mounted in the casings.
In order to avoid such disadvantages, honeycomb structures have been proposed whose outer circumferential walls and partitions in the proximity of the outer circumferential walls are made thicker, as disclosed in Japanese Laid-Open Patent Applications Nos. 55-142,189, 55-147,154 and 55-155,741 as shown in FIG. 2, which is different from a conventional honeycomb structure having uniform wall thicknesses shown in FIG. 1. However, the honeycomb structures having the partially thicker walls and partitions increase pressure losses, and what is worse still, they decrease thermal shock strength. As shown in FIG. 3, a honeycomb structure has been also proposed, which includes fine cells in the proximity of an outer circumference as disclosed in Japanese Laid-Open Patent Application No. 55-155,742. This honeycomb structure likewise increases the pressure losses. Moreover, surface areas in the proximity of the outer circumference which are difficult for exhaust gases to flow through are increased to consume a great amount of expensive catalytic metal which is disadvantageous from economical viewpoint. Furthermore, a honeycomb structure has been proposed whose outer circumferential surface is coated with a glazed layer to increase the strength as disclosed in Japanese Laid-open Utility Model Application No. 53-133,860. However, manufacturing process for the honeycomb structure are complicated to increase manufacturing cost. Moreover, there is a difference in coefficient of thermal expansion between the glazed layer and the honeycomb structure itself which lowers the thermal shock strength of the structure.